1. Field of the Invention
The present invention relates generally to fiber optic modulation and coding methods and more particularly it relates to a wavelength-sensitive receiver for converting wavelength changes to amplitude changes in an optical transmission system which uses a wavelength-encoded modulation format.
2. Background of the Invention
Fiber optic systems typically use amplitude modulation (AM) to encode data on an optical carrier signal. In this encoding method, the amplitude of the carrier waveform is modified according to the information signal that it is transporting. The receiver simply detects the intensity of the light that hits the photodetector and then uses a decision circuit to decode the signal back to the digital data (in the case of digital modulation). Any fiber optic system will have attenuation due to the losses of splices and connectors, and within the optical fiber itself. Due to these losses, a fiber optic receiver must be able to tolerate amplitude variations in the fiber optic link. This is the dynamic range of the receiver.
Another way to encode digital data on a fiber optic link is to code a “0” bit on one frequency and a “1” bit on another frequency. This binary format is referred to as frequency-shift keying. However, since this is the optical domain this could also be referred to as the wavelength-shift keying where the subcarrier wavelength determines the logical state. By encoding the data in the wavelength rather than in the intensity of the light, the sensitivity of the fiber optic link to attenuation is reduced. The receiver must be able to detect changes in wavelength rather than changes in intensity to decode the information and prevent contamination of the optical signal due to light intensity losses.
One problem with the use of wavelength-shift keying is that photodetectors, which are used to convert optical signals to an electrical output, respond to the intensity of light independent of wavelength.